Specific object recognition is processing of determining which, among objects in other images, an object taken as an image is identical to. As used herein, this processing is also referred to as image recognition. For example, such processing can be used for detection of excess or deficiency of parts, detection of counterfeits or the like, or substitute for bar-code processing, thus having a high utility. Here, “an object taken as an image” means an instance (retrieval target) included in the image as a query, and “processing of determining which, among objects in other images, an object taken as an image is identical to” is also understood as processing of retrieving an image including an identical instance from an image database in which multiple images have been registered in advance, that is, processing of image retrieval.
A method using local features is known as one of methods of the specific object recognition. In the method using local features, an image is represented by local features extracted from the image through a predetermined procedure, and the local features are compared or matched with local features extracted from another image, whereby identification (recognition) is performed. For example, local features are used in SIFT (Scale-Invariant Feature Transform) (for example, see Non-Patent Literature 1) and PCA-SIFT (Principal Component Analysis-SIFT) (for example, see Non-Patent Literature 2). Such a local feature is represented as a multidimensional vector, and therefore, is also referred to as a feature vector. The above methods have a merit that recognition with a high accuracy can be performed even when there is some occlusion or variation in an instance included in a query image and/or an instance included in an image registered in an image database, because multiple feature vectors are extracted based on local characteristics of an image.
Other literatures relevant to the present invention include Non-Patent Literatures 3, 4, and 5. Specific relevancy between the present invention and each of the literatures will be described later.